Delivery devices for administering a beneficial agent to a biological, fluid environment of use are known to the prior art. See, for example, U.S. Pat. Nos. 5,137,727; U.S. Pat. No. 5,174,999; and 5,238,687.
These devices comprise a housing including fluid-impermeable first wall section and a fluid permeable second wall section. An active agent is enclosed within the first wall section. An expandable driving member is enclosed within the second wall section. A partition member is between the beneficial agent and the expandable driving member. An exit passageway is formed in the fluid impermeable first wall section. As fluid is imbibed through the fluid permeable second wall section, the driving member expands within the second wall section, pushing the partition member which forces the active agent through the exit passageway.
The delivery devices described in the above patents operate successfully for their intended use and they can deliver many beneficial agents for their intended effects. Now, it has been observed that their use can be limited because difficulties associated with shortening the start-up time for delivering the active agents from the device.
Implanted devices are continually exposed to biological fluids naturally present in the body. Fluids are imbibed across the fluid permeable wall sections containing the expandable driving member. When the partition layer or expandable driving member has occupied the entire volume defined within the first and second wall sections, the expandable driving member continues to expand. Pressure may accumulate within the device until it is sufficient to force a separation of the first wall section from the second wall section. Thus there is a need for a delivery device that is essentially free of the problems associated with the prior art and that, if such an implantable delivery device is provided, it would have a practical application in the fields of human and veterinary medicine particularly in the breeding and management of farm animals.